Not usual as it happens at all ages, including Masters
Have a look at the intensity of training
The old adage of training harder than you play still applies
Ensure that your small sided games (SSG) are competitive
Overloading so one side has more players
Any drills you employ should start off non competitive so the concept is delivered
Then add defenders or attackers into the drill
Vary whether the attacking or defensive unit is overloaded
Wombat, current New Zealand Over 35s Coach

I'm guilty of this. my coach as well a I always seem befuddled at how i can perform so strongly and well at training.... and then be totally lost in a match. The only thing i can think of is.... the pressure in training doesn't get to me like it does in a match and i panic

The obvious first question is whether your training replicates the environment a player is experiencing in a competitive match in terms of intensity, as others have mentioned already.

The second question is whether the player is playing in an environment in which they feel safe and secure to make mistakes. An important part of developing a player is helping them to understand (and believe!) that their inherent worth as a person isn't tied to their performance on the field, or the result of a game.

Once a player understands that, it takes their thought process away from comparing (I'm better than her because I beat her) and towards competing (I'm going to try and be better than I was yesterday).

Some great insights here, thank you!